Tuesday, May 3, 2011

Reflections on Yosemite and Mass Wasting

OK, not philosophical reflections, but literal reflections. One of my favorite spots to visit on the floor of Yosemite Valley is the Mirror Lake area, due to the relative peace and serenity brought about by the lack of cars and other urban distractions. Just people walking about quietly. Mirror Lake is not exactly a lake, and not exactly a river, and for a good part of the year it is entirely dry.

Tenaya Creek is a relatively minor tributary of the Merced River, but Tenaya Canyon is nearly as deep as the Grand Canyon. That would seem to be a bit of a mystery, since the size of glaciers in a canyon can be roughly comparable to the size of the watershed, and it looks like there was a very large glacier here. Studies of the glacial topography help to explain the disparity: large masses of glacier ice spilled over the divides from the Tuolumne River drainage, adding to the ice load in Tenaya Canyon. As big as they were, glaciers never overrode the summit of Half Dome, which looms thousands of feet above the lake. They instead undercut part of the base of the dome, taking advantage of joint patterns in the granitic rock to quarry rock away from the 700 foot-high face.Mirror Lake developed several thousand years ago when a large rockfall collapsed off the north wall of the canyon, damming up Tenaya Creek. The lake mostly filled with sand and was turning into a meadow, but heavy riverflows during spring runoff scoured out the river channel and left some very wide pools that are excellent for photographing (and just appreciating) the cliffs and domes that are reflected from above.

Lest one thinks that the rockfalls in Yosemite are a thing of the past, a large rockfall occurred here just a few years ago (see some posts on the Ahwiyah Point fall here and here). Ahwiyah Point is the prominent summit just east of Half Dome (below). The rockfall destroyed hundreds of trees, and a hiking trail, which is still closed to entry due to continuing danger from falling rocks.

Mirror Lake is not hard to get to (a tram stops just about a mile away), and one can even ride a bicycle most of the way up. There is a small network of trails around the pools, and the main trail climbs out of Yosemite at Snow Creek. It is a very beautiful corner of a very beautiful valley. Don't miss it if you get a chance to visit!

4 comments:

KC
said...

Loving this series on Yosemite. One question I have is about other Sierra Yosemites. Where are they? I know Hetch Hetchy, but where are the other similar valleys (aside from "little Yosemite" in Sunol Regional Park)? Done a fair amount of Sierra backpacking but have yet to see something similar.

Not to belabor the point, but I reread my Muir and he notes, "several other yosemites have been discovered in the Sierra that occupy the same relative positions on the Range and were formed by the same forces in the same kind of granite." I can think of places, such as the Merced River Canyon near Triple Peak Fork, with high granite walls, but can't think of valleys aside from Hetch Hetchy similar to Yosemite. Much appreciate any answers, especially since I am getting my summer backpacking itinerary together now.

Hi KC, thanks for the comments! There is really no place quite like Yosemite, but Kings Canyon comes to mind, with several towering cliffs and a downstream section more than 7,000 feet deep. The North Fork of the Kings has the Tehipite Valley, with 3,000 foot Tehipite Dome (I haven't been there yet, pretty isolated). Tokopah Valley in Sequoia has some nice cliffs. Kern River Canyon is deep and straight, but I haven't seen it up close either (looked down in while approaching Whitney many years ago. The only other place that comes to mind is the Balloon Dome area on the San Joaquin River.

Garry, thanks for the locations. I've heard about the other Yosemites all my life (figured Kings Canyon would be one of them), but have yet to see a list. Since I've done a fair number of trips in the northern Sierras, it makes sense to me that south of Yosemite is where the other Yosemites are. That being said, there are some pretty amazing places up here, including the area around Noble Canyon near Ebbetts Pass, that may not be in the list, but almost as beautiful both geologically and aesthetically.

About Me

I am a teacher of geology at Modesto Junior College and former president of the National Association of Geoscience Teachers, Far Western Section. I have led field trips all over the western United States, and a few excursions overseas, but my homebase is the Sierra Nevada, the Great Valley, and the Coast Ranges of California.